Earth boring or tunneling has become a popular and important technique for excavation, particularly that associated with the laying of underground cable or pipe. Urban growth and exploding technology have both contributed to the need for subterranean excavation; for example, the laying of additional or replacement sewer lines in a city or the installation of telephone or other communication lines is becoming increasingly necessary in most urban centers. Yet, it is neither convenient nor efficient to perform this excavation from above the ground, tearing up existing roadways, disrupting traffic flow, and paying the enormous costs associated with these endeavors. Rather, underground boring or tunneling has emerged as a viable and highly desirable alternative.
The allure of subterranean boring to the contrary notwithstanding, these approaches have to date been severely limited in many ways. Most notably, the tendency for the boring tools to move off course or otherwise wander erratically has handicapped broad applicability or wide acceptability of this approach. Thus, boring 20 or 30 feet across a roadway course has been found to be an efficient application of this technology; however, boring several hundred feet along a roadway course has heretofore been found to be too great a challenge for existing technology. As the tool, whether auger or impact type, proceeds along its earthen path, it is subject to diversion from a desired or predetermined course as the device strikes rocks or other obstructions, moves through zones of differing soil density, perhaps traverses a path through or beneath the watertable, or encounters similar underground conditions. Sometimes these deviations can be acceptable, particularly where the course is short. Where a tool is required to progress several hundreds of feet, however, even small angular deflections can amount to substantial departures. In these latter cases, it is not simply an inconvenience of missing the mark alone which plagues the operator; sometimes the tool itself can be lost and require major excavation to retrieve it or suffer its forfeiture.
Scores of proposals have surfaced to monitor the vertical disposition of a boring or tunneling tool. In instances where large auger-type tunneling tools are utilized, creating caverneous bores through the earth, transits or other conventional line-of-sight procedures have been adopted with reasonable success. More recently the availability of laser technology has improved the accuracy of these techniques. Workable as they are, however, these approaches begin to falter as the diameter of the bore or tunnel moves down to the scale of about two feet or less, typically considered the realm of microboring.
The present inventor has successfully proposed an elevation monitor for measuring the percent of grade of an earth boring tool. Referring specifically to U.S. Pat. No. 4,438,820, the system includes, inter alia, a grade sensor having a reservoir containing a liquid medium at a predetermined level, a light source for transmitting light through the liquid and an opposed light detector for determining the intensity of light transmitted through the liquid. As the tool changes orientation the level of fluid changes proportionately as well. In turn the transmittance of light through the liquid becomes greater or lesser, the detection of which is a direct indication of the pitch or grade of the tool. That system further includes means for correcting the path of the boring apparatus in order to maintain it on a predetermined desirable course, in the nature of skiis or runners.
Others have suggested the use of fluids such as water to establish a gauge for measuring elevation, relying on the principle that the liquid will seek a common level within an open circuit. Exemplary of that approach is the device disclosed in U.S. Pat. Nos. 3,851,716 and 3,939,926. In those cases a fluid circuit, including a sensing member carried on the casing of an auger, is charged with water through a series of valves. The sensor is placed in fluid communication with a gauge centered at or about a reference level along the path of interest. Venting the system (i.e., bleeding off an excess fluid head) establishes a point of reference at the sensor while the level of fluid in the gauge conforms to that level simply as a matter of fundamental physics. In turn, the operator may observe the rise and fall (within rather confined limits) of the fluid column and hence, indirectly, the rise and fall of the auger itself.
Representative of other systems perhaps adaptable to this same end are those disclosed in U.S. Pat. Nos. 3,657,551, 4,129,852, and 4,154,000. While these devices are not necessarily disclosed to be useful in this particular environment, the same generally relate to slope indicators of various designs to which general reference is made for the sake of completeness.
While considerable attention has been paid to the monitoring of vertical disposition of a boring tool, few meaningful advances have taken place respecting the detection of horizontal deviations of a boring member from a predetermined or desired path. The complexity of that objective has, to date, virtually precluded meaningful, economically efficient approaches.
One proposal which has been noted with more than casual interest by those in the field has been advanced by the National Gas Research Institute. A boring tool is provided with a tightly wound wire coil which receives low frequency current, establishing a magnetic field centered about the tool. A receptive or detector coil is disposed in the pit at the entrance to the bore hole, this coil being in the shape of a "t" to present both vertical and horizontal receiving arms. The detector coil is thought to be able of detecting the field emanating from the moving boring tool at a range of up to about 100 feet. With the strength of the field falling off in relationship to an inverse square law, however, one can appreciate the enormous power requirement this approach necessitates. To date it has not received the kind of acclaim in the field that forebodes a promising development of this technology; nonetheless, it remains the most viable of known techniques to monitor horizontal disposition of a boring tool.
Monitoring the spatial orientation of a remote boring tool is a worthwhile endeavor at least to the extent that it allow an operator to locate a tool should it stray from a predetermined course without the need for major excavation. Better, however, is the ability to make corrections to the path of a moving tool in order to maintain it on the desired course in response to the detected deviations one may ascertain. Again, in the realm of the large tunneling devices, the ability to detect and then correct path departures is a relatively straightforward task. Note, for example, the approaches suggested in U.S. Pat. No. 4,042,046, with which one may profitably compare, for example, the aforementioned '716 and '926 patents as well as U.S. Pat. No. 2,946,578.
Moving to the smaller scale or microboring tools, the problem is exacerbated by the reduced sizes involved. The present inventor's aforementioned '820 patent discloses a ski arrangement of members pivotably connected to the boring casing and operable to guide the same by exerting a pressure against the earthen bore hole. Other approaches to this problem of steerage of microboring tools have not been forthcoming.
In view of the increasing importance of earth boring in general and microboring in particular, the art has not advanced concomitantly in the field of monitoring and controlling such devices over long courses such as those of 100 feet or more. The need thus exists for improved vertical and horizontal monitoring systems to detect spatial orientation of the boring tool as it traverses its path and for ascertaining deviations in that path from a predetermined or desired course. A need likewise exists for guidance mechanisms to take full advantage of the ability to monitor the path of a moving boring tool accurately and make suitable corrections to ensure the tool maintains a desired, predetermined course.